In a known device for ultrafiltration (DE-OS 26 53 875), the suggestion is made that the slot height be held as small as possible to avoid a formation of a covering layer in order to also keep the developing filter covering layer as small as possible in this manner. This is to be achieved by designing an elastic, impermeable pressure pad between two filter plates with filter medium resting thereupon which pad forms one wall of the slot-shaped filter chamber. The pressure pad should form a pressure-compensation body which is deformed during the flowthrough of the liquid according to the pressure conditions on its surface. This should generate the same local flow resistance over the entire liquid.
It turned out in practice, however, that preferred flowpaths develop in certain areas and the excessive formation of a covering layer can be only partially limited.
It is also customary in filter elements operated according to the cross-flow method (DE-OS 24 41 249) to partially remove a filter covering layer which develops on the microporous filter membrane by loading the filter element from the filtrate side with filtrate or a rinsing agent and subsequently rinsing away the filter covering layer. This has the result that a constant changing must be performed during the actual filtration operation between filtration and backwash with the filtrate already gained and the backwash procedure is only worthwhile during a general cleaning with neutral rinsing agent. In both instances of backwashing, the filter element is therefore also stressed against the actual direction of filtration.
It is also known (DE-OS 34 11 471) that the built-up covering layer in hose filters can be broken down by the hose filters extending freely through the filtrate area and in that their cross section collapses in a reversibly flexible and irregular manner in the case of excess pressure on the filtrate-side. The filter elements are heavily stressed especially by this pulse-like collapsing, which can result in ruptures.
Filtration according to the cross-flow method is becoming more and more significant in the beverage industry. The liquids to be filtered are loaded more or less heavily with finely distributed components. In the filtration of unclarified wines in the cross-flow method, a very rapid and strong covering of the filter membranes with a filter covering layer occurs, even if the attempt is made to delay this flow speeds.
Similar problems occur if a specimen is to be taken from a circulating fermenter broth. The fermenter broth conducted over a filter element in accordance with the cross-flow principle covers the filter membrane very rapidly with a filter covering layer. The fermenter filtrate drawn off on the filtrate side thus frequently does not reflect the actual state of the fermenter broth since the cells from the cell culture contained in the filtrate do not stem from the actual fermenter broth, which is conducted past the covering layer, but rather are old cells or damaged cells which are already located in the filter covering layer.